1. Field of the Invention
This invention relates generally to a heat-resisting and abrasion-resisting tuyere for a blast furnace made of copper or copper alloy and a method of surface-coating the same, and more particularly, to a blast furnace tuyere made of copper or a copper alloy with a specific coated layer thereover capable of sufficiently resisting heat and abrasion attacks caused by the splashing of molten metal and slag. The invention is also concerned with a method of surface-coating the tuyere with advantages both in process and in economy.
2. Prior Art
Prior to this invention, the present applicant has made a copending application for an invention regarding a heat resisting copper or a copper alloy product having a specific surface-coated layer thereon by which marked improvements were made in high temperature oxidation resistance, corrosion resistance, abrasion resistance and thermal shock resistance. This prior invention was also directed to a method of surface-coating the product. This prior invention was made public with Japanese patent publication No. 105739-1974 (patent application Ser. No. 17649/1973) which is herein incorporated by reference.
In this invention, the aforesaid specific surface-coated layer includes metallic oxide powders of high melting point such as Al.sub.2 O.sub.3, ZrO.sub.2, TiO.sub.2, Cr.sub.2 O.sub.3, etc. and a fused Al layer which is carried in or carried in and simultaneously absorbed into and embraced by the oxide powders and the base metal present directly below the coated layer formed of a diffusion-penetration layer of Al-Fe. With the layer construction as above, the copper or copper alloy product is increased in its heat, abrasion and other resisting properties thus adapting it for use with various members such as the wall, nozzle, etc., in a blast furnace, converter furnace and the like.
Since the contents of the previous invention have been disclosed in the Japanese patent publication, more detailed explanation of the previous invention other than the above is omitted herein. A description will now be given of the reasons that have made it necessary to make the improvements as set forth in the previous invention.
(A) Heat resistance and abrasion resistance strongly demanded of the tuyere in the furnace has made it insufficient to have recourse exclusively to the coating means provided by the previous invention and has necessitated further increases in both properties. One means conceivable for this purpose by those skilled in the art is to increase the concentration of Al of the constituent parts of Al in the coated layer. Too great a concentration (for example, 40%), however, of Al in the coated layer causes thinning and cracking in the surface of the diffusion-penetration layer. This makes it impossible to attain the expected advantages due to an increase in concentration. In order to prevent such thinning and cracking, there is no alternative but to limit an Al content up to at most, approximately 28% (by weight). Even in this case, the hardness necessary for the surface of the tuyere cannot be obtained. Herein lies one stumbling block to which the problem of increasing the concentration of Al of coated layer is always brought.
On the other hand, when Al powder alone is used as was the case with the previous invention, there are two modes of diffusion-penetration by Al powder; one mode being that the Al powder itself is fused to the surface of the base metal and makes contact diffusion from the fused surface, and the other mode being that of vaporizing Al so as to diffuse through the base metal. Despite the fact that the latter method produces a product which is originally stronger than that produced by the former method in diffusion effect, the latter method is less effective in causing diffusion through the base metal under the conditions for treatment according to the previous invention. Therefore, sufficient treatment cannot be expected from a diffusion-penetration mechanism based exclusively on the Al powder. Prolongation of the time for treatment to improve this as much as possible results in disadvantages in the treatment cycle. Even if the increase of quantity of diffusion of Al should prove successful, for example, when the quantity of Al becomes 30% (by weight), an undesirable effect of thinning and cracking is likewise produced on the surface of the base metal. In this manner, the advantages of the previous invention still leaves a difficult problem that cannot be solved either by using various coating material compositions or by treatment conditions.